Uses of Dorr and Other Thickeners.—Outside of the separation use to which these devices can be put is their employment in what is termed "counter current washing." To show the extractive and recovery possibilities a hypothetical computation based on three tanks will be made. It is assumed for convenience that the tanks hold 110 tons of liquid and solid in the proportion by weight of 10 parts of liquid to one of solids. It is desired to save the liquid which is valuable and reject the solids with the minimum of valuable liquid. Let it be further assumed that the solids will settle to a ratio of liquid to solids of 1 :1 in each tank.
FIG. 14.—Diagrammatic sketch, showing thickening with, four tanks.
The mode of employing the three tanks would be to pump the underflow of the first tank to the second and dilute it with the overflow of the third. The underflow of the third tank goes to waste. Its feed is the underflow of the second tank plus the necessary wash water. The overflow of the first tank is finished liquid and goes to a storage tank from whence it may be drawn off for use or evaporation, crystallization or any operation to render it a usable or commercial product. The overflow of the second tank goes back into the main plant and after being used in making fresh solution will come back to the decantation apparatus again. The flows in the various tanks are indicated diagrammaticaUy in Fig. 14 in which four tanks are engaged.
Since the ratio of liquid to solids is 1 :1, 10 tons of 100 per cent liquid is removed with the solids from the first tank and passes to the second. Ninety tons of finished liquid pass at once from the first tank to the storage tank and there is consequently a 90 per cent saving effected in the first tank The 10 tons of 100 per cent liquid pass into the second tank along with the overflow of the third tank. This overflow consists of wash liquid of zero strength mixed with the liquid of very much less than 100 per cent strength which comes from the underflow of the second tank. If S" is the strength of the liquid in the second tank expressed in percentage of that in the first tank and S1" that in the third then evidently
10 X 100 + 90S'"
= S" and
10£" + 90 X 0
The first expression boils down to Strr equalling 1.099 when the value of S" equals 10$"' obtained from the second equation is substituted. Consequently the strength of solution in the second tank is 10.99 and in the third 1.099. If two tanks only were employed the underflow of the second would carry to waste 1.099 tons or 1.099 per cent of the weight of the 100 per cent liquid run into the first tank or two tanks would effect an extraction of 98.90 per cent and three tanks effect a 99.89 per cent extraction. Having completed the computation it is only necessary to turn the tonnage figures used into rates of feeding to obtain the idea of continuous operation. This mode of decantantation arose in the cyanide process for extracting gold from ores. Of late years its use has been extended into other fields such as the continuous extraction of caustic soda from lime mud, in chlorine manufacture, as an auxiliary in the use of adsorbents such as silica gel, etc. (See also p. 354.)